[Enhancements] restore dbcachesize with the backend bdb (ITS#2372)

Full_Name: Luc Saillard
Version: 2.1.15
OS: Linux
URL: http://luc.saillard.free.fr/patches/openldap/openldap-dbcachesize_bdb.patch
Submission from: (NULL) (62.147.141.167)
When using db_stat -m to see if my db cache size is correct i've only 70% hits.
My first patch have hardcoded value, so i've use the same code from the backend
ldbm to set the cachesize. Please change the default value, this is 2Mbytes by
default.